Previous computer input devices, such as mice, include rotatable balls mounted within a housing, yet rotatably engaging a surface. As the housing of such a mechanical mouse translates across the surface, the ball rotates within the housing, engaging horizontally and vertically situated wheels that rotate against the ball, thereby indicating horizontal (e.g., side to side or x-direction) and vertical (e.g., back and forth or y-direction) movement of the mouse across the surface. When the device is lifted from the surface, hereinafter referred to as “lift-off” or the detection of an “off-surface” condition, the ball stops rotating and the horizontal and vertical movement information provided by the wheels stops. This feature is particularly useful to a user who has reached a point where the device can no longer move with respect to the tracking surface, but the user would like to continue tracking in that particular direction on a display. By lifting the device off of the tracking surface, the user can reposition the device, while a display cursor remains stationary because tracking is suspended during lift-off. When tracking resumes, horizontal and vertical wheel rotation translates into a displayed visual image of the display cursor responding to movement of the device. Because such devices have a moving ball passing through a hole in the housing, such devices may become contaminated with dust and dirt, yielding inaccurate or intermittent cursor tracking. Moreover, the tracking surface and ball require sufficient friction between the two to cause the ball to rotate when the housing translates over the surface. To help provide such friction and minimize device contamination, specialized tracking surfaces (e.g., mouse pads) are typically used. Thus, a major limitation of such a device is that it requires a tracking surface with particular characteristics, such as adequate friction and cleanliness, which are not readily found on all surfaces that would otherwise be useful for tracking.
Building upon these primarily mechanical tracking devices, optical tracking devices have become available. Such devices optically track movement relative to a surface, rather than mechanically as with the devices described immediately above. These optical tracking devices may avoid some of the drawbacks associated with the mechanical devices described above. In particular, optical devices typically do not require contact between wheels and a movable ball, which acts as a common collection point for dust and dirt. Instead, the movable ball may be covered with a distinct pattern. As the ball rotates over a surface due to movement of the input device, photodetectors facing another side of the ball collect information about the movement of the ball's distinct pattern as the ball rotates. A tracking engine collects this information, determines the direction the pattern is translating, and translates the cursor on the display, as similarly described above. Lift-off detection is performed as discussed above, such that when the device is lifted, the ball stops moving and the device stops tracking. These devices offer benefits over previous designs by eliminating moving parts (the wheels) and changing the ball detection interaction from mechanical to optical. However, such devices lack the ability to track on any surface, requiring a suitable frictional interface between the ball and the surface. Moreover, these devices still require one moving part, namely, the ball. Moreover, such devices may become contaminated with dust and dirt, thereby possibly obscuring the detected pattern and/or creating fixed pattern noise on the detected image. Finally, aliasing artifacts may also cause the cursor to skip, rather than move fluidly.
Still other optical devices place a pattern on the tracking surface (e.g., a mouse pad), rather than on the rotatable ball, thereby using the mouse pad to generate optical tracking information. Although such devices are able to eliminate the moving ball, they are less universal by requiring a specific tracking surface to operate.
Other more recent optical tracking devices eliminate the need for a patterned ball or mouse pad. One such device utilizes an LED to project light across the tracking surface at a grazing angle relative to the tracking surface. The mouse then collects tracking information by detecting the differences between the illuminated high points in the surface texture and dark shadows cast by the high points, which appear as dark areas. Such an LED device eliminates the moving ball of previous devices, and is useful on a variety of surfaces. However, smooth surfaces with little color variation, such as surfaces with a fine microfinish similar to glass or clear plastic, may prove difficult to track upon. Like the devices discussed in the paragraph above, these devices need not specifically address detection of an off-surface condition when the device has been removed from the tracking surface. Instead, they utilize focused optics and are thus inherently unable to track if the device has been lifted a significant distance from the tracking surface.